Photocurable resin coating agents have been extensively studied recently in the field of hard coatings to provide a lighter and less expensive alternative to metals and glass. In particular, many coating agents that are UV-curable with a photoradical generator have been reported (Patent Literature 1). Unfortunately, however, curing methods using radicals tend to allow curing to be inhibited by oxygen, so that curing tends to be insufficient around the interface between the coating agent and air.
Coating agents that mainly include a compound containing a cationic curable functional group such as a hydrolyzable silyl group or epoxy group and are curable with a photoacid generator have also been reported (Patent Literatures 2 and 3). When curable compositions mainly including a compound containing a cationic curable functional group such as a hydrolyzable silyl group or epoxy group are photocured, curing is not inhibited by oxygen in the air and the entire coating is instantly cured, achieving sufficient hardness and sufficient scratch resistance.
In particular, a siloxane bond, which is formed by cationic curing of a hydrolyzable silyl group, has drawn attention because it tends to provide high hardness and high scratch resistance. However, when a hydrolyzable silyl group is used for curing, hydrolysis/condensation polymerization is accompanied by shrinkage on curing, causing cracks and warpage. In this context, various attempts have been made in which a hydrolyzable silyl group of a silane compound containing an epoxy group, vinyl group or the like is hydrolyzed/condensation polymerized with an acid catalyst such as hydrochloric acid, acetic acid, or formic acid in advance, and the silane compound is photocured via the epoxy group, vinyl group or the like. An epoxy group is particularly often used to suppress shrinkage on curing after photocuring. However, the hydrolyzable epoxy group is hydrolyzed in the presence of the acid catalyst described above and the resulting coating unfortunately fails to have sufficient physical properties. In view of application to the entire panel of devices such as smartphones or tablets, the coating needs to be excellent in hardness and scratch resistance and it is necessary to allow as many epoxy groups as possible to remain so as to achieve high crosslink density.
In addition, laminates having a layer formed from a polyfunctionalacrylate curable composition exhibit warpage when left under high temperature and high humid conditions.